I read grounding one of them helps with the 3700 problem, but which wire?

 

I don't think it was the boost sensor wire that is supposed to be grounded. I know that regrounding the standard ECU grounds will help if not totally get rid of the 3700-3800rpm studder of when the secondaries come online. Check my webpage for the wiring diagram in the FSM. It will show you there which wires are grounds.